1. Field of the Invention
The present invention relates to a hydraulic circuit system for hydraulic working machines such as hydraulic excavators, and more particularly to a hydraulic circuit system for hydraulic working machines provided with a safety control device, e.g., a lock valve, which is operated upon a gate-lock lever manipulated by an operator when the operator has no intention of carrying out work, for cutting off pilot primary pressure from a control lever unit.
2. Description of the Related Art
A hydraulic circuit system for hydraulic working machines such as hydraulic excavators generally comprises a variable displacement hydraulic pump driven by a prime mover (engine), and flow control valves for supplying and returning a hydraulic fluid from the hydraulic pump to and from actuators. When any of control levers of control lever units is operated, a command signal in the form of pilot pressure, for example, is applied to the associated flow control valve, whereupon the flow control valve is driven to shift in position to operate the actuator.
Also, the hydraulic circuit system includes a regulator as tilting control means for controlling a tilting of the hydraulic pump and hence controlling a delivery flow rate of the pump. There are various types of regulators. One example of regulators which has an input torque limiting function is designed to receive the delivery pressure of the hydraulic pump, and when the pump delivery pressure becomes high, to make smaller a pump tilting to reduce a delivery flow rate so that the pump absorbing torque will not exceed the output torque of the prime mover driving the hydraulic pump. This surely prevents the prime mover from stalling even when the pump delivery pressure is so high. The prior art related to the regulator having an input torque limiting function is disclosed in, e.g., JP, Y, 62-26630.
In a hydraulic circuit system having a center bypass line by which flow control valves of the center bypass type are connected in series, a negative control type regulator is employed which detects a center bypass flow rate in terms of pressure and controls the pump tilting in accordance with the detected pressure. The negative control type regulator is designed to make smaller the pump tilting to reduce the delivery flow rate when the center bypass flow rate is large and the detected pressure is high, but to make larger the pump tilting to increase the delivery flow rate when the center bypass flow rate is small and the detected pressure is low, thereby delivering a pump flow rate depending on the flow rate demanded by the flow control valve to reduce an energy loss.
Also, in the negative control type regulator, the tilting of the hydraulic pump is generally controlled such that when the control levers are not operated and the flow control valves are in the neutral positions, a certain flow rate larger than a minimum flow rate is maintained as a stand-by flow rate to improve response of actuators at the start-up of operation. The prior art related to the regulator performing the negative control and setting the stand-by flow rate is disclosed in, e.g., JP, U, 6-28304.
Meanwhile, in hydraulic working machines such as hydraulic excavators, a lock valve is provided as safety control means to keep the machine from coming into operation even if a control lever is erroneously touched when the operator has no intention of carrying out work in such an occasion as getting off the machine. The lock valve is disposed in a pilot line through which the pilot primary pressure is supplied from a pilot pump to a pilot valve of a control lever unit. When a gate-lock lever is operated, the lock valve is shifted to cut off the pilot primary pressure, whereby the pilot secondary pressure, i.e., the command pilot pressure, is output in no way from the pilot valve even with the control lever moved. As a result, erroneous operation of the machine is prevented. The prior art related to the lock valve is disclosed in, e.g., JP, U, 5-57052.